The maintenance of lipid droplet and energy homeostasis and the impact of ageing during hepatocyte polarization

Abstract

Hepatocytes have a well-defined polarized morphological structure. Establishment and maintenance of polarity is crucial for the function and viability of hepatocytes. Many disorders, both genetic and acquired, and drugs can impair polarization resulting in liver dysfunction and injury. Hepatocyte polarization is an energy-dependent process. Hence, cellular energy metabolism can significantly affect the polarized morphology and function of hepatocytes. An in vitro study has shown that there were diminishing numbers of lipid droplets in correlation to increased autophagy, ATP levels and mitochondrial membrane potential and the establishment of polarization. It is well established that lipid droplets supply fatty acids to mitochondria for β-oxidation and ATP production, where insufficient number of lipid droplets can impair cellular function and elevated lipid droplet accumulation can cause lipotoxicity. Investigation by pharmacologically inhibiting different pathways involved in maintaining lipid droplet homeostasis revealed that primary hepatocytes cultured in a collagen sandwich configuration were dependent on lipolysis of lipid droplets to release fatty acids, regulated via perilipin 2 and 5, which were then chaperoned by fatty acid binding protein (FABP1) for energy production during polarization. In addition, changes to carnitine palmitoyltransferase 1 (CPT1) expression suggested a compensatory mechanism to overcome the disrupted fatty acid levels and to maintain energy homeostasis. Conversely, autophagy of lipid droplets, also known as lipophagy was dispensable for breaking down lipid droplets. However, autophagy of cellular materials to provide triglycerides and cholesterol esters for lipid droplet formation and maintenance of fatty acid pool, together with cholesterol biosynthesis/esterification and de novo fatty acid synthesis were essential for energy production and polarization. In summary, multiple cellular pathways co-ordinately contributed to lipid droplet homeostasis to sustain fatty acid levels and β-oxidation during hepatocyte polarization. Ageing in the liver is associated with decline in hepatic volume and blood flow, reduced drug metabolism and overactive inflammatory responses. However, it is unknown how ageing impacts hepatocyte polarization. In this study, we have shown that polarization was accelerated in old hepatocytes despite decreased mitochondrial function, increased lipid droplet accumulation, and reduced AMPK activation compared to the young. Further examination of the mammalian target of rapamycin complex (mTOR) pathway showed increased S6 kinase expression (downstream effector of mTOR) associated with senescence and senescence-associated secretory phenotype (SASP) in old hepatocytes, which may have contributed to accelerated polarization together with loss of lipid droplet and bioenergetic homeostasis. We have further confirmed that there was increased deposition of collagen, dilation of bile canaliculi and overall disruption to biliary structure with less tight junctions intact in the old human liver specimens through immunohistochemistry and transmission electron microscopy. There were also morphological changes to mitochondria, larger lipid droplets, and accumulation of lipofuscins in old liver specimens. Overall, there were age-related changes to polarized morphology in hepatocytes cultured in collagen sandwich configuration and in human liver, which may be associated with elevated risk of adverse drug reactions, hepatocellular carcinoma, atherosclerosis, non-alcoholic fatty liver disease, atherosclerosis, and diabetes during ageing.